In addition to carrying linear momentum, a beam further carries angular momentum. The angular momentum includes spin angular momentum (SAM) and orbital angular momentum (OAM). The SAM is usually related to spin of a photon, and such a relationship is embodied as circular polarization. A value of the SAM may be ±ℏ. In contrast, the OAM is related to a spiral phase wave. In 1992, Allen et al.
found a beam with an exp (ilφ) spiral phase factor by means of experiments and researches. The OAM carried by each photon of the beam is lℏ, the value is 1 times the SAM, and φ is an azimuth.
In recent years, application of the OAM in the field of communications has attracted wide attention. Different from the SAM with only two values ±ℏ, lℏ of the OAM may be any value within (−∞, +∞), and different OAMs can be easily distinguished. Therefore, a communication capacity and spectral efficiency can be greatly increased by performing communication based on the OAM. As a result, generation and detection technologies of a vortex beam with OAM have attracted increasing attention from people.
Research institutions from home and abroad have made extensive and careful researches on the field of vortex beam generation, and manners of generating a vortex beam vary. For example, the vortex beam is generated by using a V antenna array.
A V antenna in the V antenna array includes a metal V-shaped structure and a silicon substrate, and the metal V-shaped structure is grown on the silicon substrate. When incident light is emitted onto the metal V antenna, surface plasma oscillation is caused, and scattered light has a particular amplitude and phase. The amplitude and phase of the scattered light are related to an arm length and an included angle of the metal V antenna. By designing the arm length and the included angle of the V antenna, scattered light resulting from a Gaussian beam emitted onto the V antenna array may have the OAM, thereby generating a vortex beam.
A structure of a V antenna that can generate the vortex beam is relatively small. This is not conducive to production. In addition, because the V antenna is of a metal structure, the metal structure absorbs incident light, thereby resulting in a relatively large loss.